(1) Field of the Invention
This invention relates to an ultrasonic pulse Doppler blood flow meter, and more specifically to the characteristic control of a receiving circuit.
(2) Description of the Prior Art
An ultrasonic pulse Doppler blood flow meter, which measures a blood flow rate and its distribution by transmitting an ultrasonic pulse wave to living body tissue and by receiving a reflected wave from a blood corpuscle, is currently attracting attention. The equipment currently being used requires many manual adjustments and is not easy to operate.
FIG. 1 is a block diagram of a conventional ultrasonic pulse Doppler blood flow meter. In FIG. 1, 1 is a master oscillator, 2 is a transmitting timing generator which generates a transmitting timing signal by dividing an output of the master oscillator, 3 is a transmitting amplifier which generates a pulse or burst transmitting signal, 4 is a transducer (ultrasonic wave probe) which generates an ultrasonic wave to a living body in accordance with the transmitting signal and receives a reflected wave from the living body, 5 is a receiving amplifier which amplifies the reflected wave signal received by the transducer and 6 and 7 are real (R) and imaginary (I) Doppler element detectors. The detectors 6 and 7 include mixers 61 and 71 which receive the cosine and sine signals, allow a phase difference of 90.degree. from the master oscillator and carry out orthogonal detection, low-pass filters (LPFs) 62 and 72 and sample and hold circuits (S/H) 63 and 73. The detector 6 detects a real element of the Doppler signal reflected from the specified depth (distance between the probe and the location generating the reflected wave), while the detector 7 detects the imaginary element of the Doppler signal. High-pass filters 8 and 9 (HPFs) eliminate a low frequency Doppler element, generated by the wall of the heart, at the outputs of the detectors 6 and 7. A Doppler analyzer 10 is provided with an A/D converter and a digital processor which analyzes frequency by a fast Fourier transformation (FFT) at the outputs of the filters 8 and 9 (the Doppler element indicating a blood flow rate). A display 11 is used for indicating the result of the analysis. A sample pulse generator 12 generates the sample pulse to the sample hold circuits 63 and 73 in accordance with a position designation signal and the output of the transmitting timing generator 2. As explained above, since the cosine and sine reference signals, allowing a phase difference of 90.degree., are input to the mixers 61 and 71 from the master oscillator and orthogonal detection is carried out by the detectors 6 and 7. A gain of the receiving amplifier 5 can be adjusted by a variable resistor 13 provided at the operation panel for gain control.
In the Doppler blood flow meter of this type, since the HPFs 8 and 9 have an upper limit input level (about .+-.10 V), an output level of the HPF becomes low when the Doppler signal contains a low frequency element due to movement of the wall of the heart and, therefore, the Doppler analyzer 10 in the next stage must be highly accurate. However, there are other problems: (1) brightness of the display 11 must be adjusted; (2) a cut-off frequency of the HPFs 8 and 9 must be changed in accordance with a blood flow rate; and (3) a gain of the receiving amplifier must also be changed in accordance with the level of the receiving signal.